Rake receivers are widely used in CDMA (Code Division Multiple Access) radio communication systems. A rake receiver seeks to synchronise a plurality of parallel receivers, known as rake fingers, to respective received signals having different path delays.
The transmitters and receivers in CDMA radio communication systems include filters, the consequence of which is the generation of spurious signals time shifted with respect to the original signal. For example, in a typical system the transmitter and receiver filters act to produce a raised cosine pulse shape in the receiver. Such a pulse shape has temporal side lobes. With raised cosine filters, the maximum of the first side lobes are normally at a distance of 1.4 chips from the main lobe. If the filters sample at 0.5 chip intervals, the closest in of the side lobes are 1.5 chips either side of the centre of main lobe and approximately 14 dB down.
The main lobe and side lobe signals appear to a rake receiver as a relatively strong signal preceded versions following shorted but more lossy paths and followed by signals following longer and more lossy paths.